Conventionally, in the field of milling of metals and the like, a rotary milling cutter such as an end mill that performs milling by rotating about a rotation axis has been used. Such a rotary milling cutter is provided with a cutting edge at its distal portion or at a side of the distal portion, and is capable of milling by rotating about the rotation axis.
This type of rotary milling is used, for example, for forming metal molds. When pocket milling is performed on metal blocks to be used as metal mold materials, rough milling is performed using a large-diameter end mill for the purpose of reducing processing time. Then, as shown in FIG. 6, four corners are formed using an end mill having a diameter suitable to the desired pocket shape.
However, the above-mentioned rotary milling method that uses a conventional end mill and the like has the following problems.
Specifically, the above-mentioned rotary milling cutter such as an end mill performs milling by rotating about the rotation axis. Therefore, as in the above case of forming a metal mold, when a block metal piece is pocket milled with a predetermined depth, the size of each angle R of the four corners of the pocket will coincide with the diameter of an end mill 50, as shown in FIG. 6.
Accordingly, when intending to pocket mill and form each corner with a small R of 0.1 mm to 0.2 mm, which is close to a right angle, it is necessary to repeat milling over and over again using a small-diameter end mill with R 0.1 mm to 0.2 mm after rough milling. Consequently, there is a problem in that the time and cost required for processing will increase.
In addition, in order to mill and form the four corners with angles as close to perpendicular as possible, the diameter of an end mill needs to be as small as 0.1 mm to 0.2 mm, and thus the strength of the distal portion of the end mill will drop and the distal portion will become easily breakable during milling and the like.
Further, although several prior art documents cited below disclose milling methods in which an end mill is used, these documents do not disclose anything regarding a milling method for orthogonal milling or edge milling in which a very small corner R is formed (see for example Japanese Laid-Open Patent Application Nos. 2003-53616 (published on Feb. 26, 2003), 2000-263308 (published on Sep. 26, 2000), 2000-5915 (published on Jan. 11, 2000), 10-128610 (published on May 19, 1998), and 06-8026 (published on Jan. 18, 1994).